The human spine is a series of bony vertebral bodies separated by flexible discs called “intervertebral discs.” The discs contain a compressible, flexible jelly-like center called the nucleus pulposus. This flexibility allows the spine to bend and twist. The annulus fibrosus is a tougher, fibrous material that surrounds and contains the nucleus, which otherwise could be extruded into other parts of the body.
The human skeleton provides an architectural structure for the human body. The human spine provides not only a mechanical structure for supporting a person's weight, but also provides passages for the “wires” of the nervous system. The spine provides spacing for the nerves to travel from the brain to other parts of the body. The majority of this spacing is the spinal canal which contains and protects the spinal cord. The spine also defines, at each level of the spine, spacing for nerves roots to branch off from the spinal cord and travel to other parts of the body. Intervertebral discs between the vertebral bodies are critical in providing the appropriate amount of spacing between the vertebral bodies to allow room for these nerve roots. If the discs begin to collapse from their normal height, nerve roots may become compressed and cause pain.
The discs also help provide alignment of the vertebral bodies, keeping the spinal canal as a relatively smooth passage for the spinal cord. If the discs become misaligned, the spinal canal may become disjointed and extremely narrowed in portions. If it becomes too narrowed, the spinal cord may become compressed and cause pain. Spondylolysis is an example of one condition which can affect the spacing in the spinal canal. In spondylolysis, one vertebral body slips forward relative to another vertebral body. This can result in a narrowing of the spinal canal and compression of the spinal cord.
Other spinal disorders can also result in nerve compression. For example, degenerative disc disease (DDD) can result in a herniated disc. A herniated disc occurs when a portion of the nucleus is extruded from the disc space. This extrusion might impinge on a nerve and cause pain. Also, the extrusion of too much of the nucleus may result in a reduction in the height of the disc, and cause narrowing of the spacing available for nerve roots.
A common method of managing these problems is to remove the problematic disc and replace it with a device that restores the disc height and allows for bone growth therethrough. This results in the fusion of two or more adjacent vertebrae. The devices used in this procedure are commonly referred to as “fusion devices” or “fusion cages”.
In a fusion procedure, a surgeon first accesses the intervertebral disc space. Next, the surgeon clears out a portion of the intervertebral disc space to make room for the fusion device (or cage). The surgeon may determine the appropriate size fusion cage by using trial implants and “testing” its fit via tactile and visual assessment, often assisted by fluoroscopy. Neuromonitoring can also be used to confirm that there is no significant injury or damage to the neural structures. For example, neuromonitoring can be used to test the response and latency of the nerves which can give an indication of how healthy the nerve is. However, neuromonitoring does not determine if the procedure has created enough “space” (referred to as neural release) to relieve the compression on the nerve.